11 September 2017

Practical DC Generator

  • There are basic following elements of an electrical generator
Stationary part
  • Field system which produce magnetic field
Rotating part
  • Armature winding which produces an induced emf
Prime Mover
  • Supply mechanical power to an armature
Working of single loop generator

single loop generator

  • Figure A shows a single turn rectangular coil ABCD rotating clockwise in a magnetic field provided by main field poles.
  •  The two ends of the coils are connected to two slip-rings 'P' and 'Q' which are insulated from each other. 
  • The two brushes press against the slip - rings and their ends are connected to galvanometer G. 
position of single coil and corresponding emf

  • Figure B Shows positions of the coils and induced emf due to those positions.
  • Let us assume that the position of the coil ABCD is in vertical ( 0° angle ). 
  • The coil sides AB and CD do not cut the flux in this position but they move parallel to them therefore no induced emf in this position. 
  • Figure B shows a position of coil is horizontal ( 90° angle ). The rate of change of flux linkages with the coil is maximum in this position therefore the induced emf is maximum. The galvanometer shows a direction of induced emf. 
  • When a coil ( DCBA ) rotates from position 90° to 180°, the flux linkage with the coil sides DC and AB increases but at the same time the rate of change of flux linkages decreases. Hence the induced emf is again zero at 180° angle coil position. 
  • When a coil rotates through 180° to 270° angle, the rate of change of flux linkage increases. The induced emf is again becomes maximum but it is in negative direction at 270°.
  • When a coil rotates through 270° to 360°, the rate of change of flux linkages decreases again. Hence the induced emf is gradually decreased and becomes zero at 360°. 
  • Now, Look at the position of galvanometer and waveform obtained from coil positions. We find that current obtained from generator reverses its direction after every half revolution i.e. alternating current ( a positive cycle and a negative cycle ). It should be noted the armature winding and slip rings produce alternating current.
How conversion is done from alternating current ( AC ) into direct current ( DC ) ?
  • If the slip – rings are replaced by split rings ( or commutator ), the conversion from alternating current to direct current is possible. 
  • Each end of the coil is connected one half of a copper ring as shown in the figure C. 
  • The two copper rings are insulated from each other by thin sheet of mica. Each end of copper ring – brush is connected to galvanometer. 
single loop generator
  • Figure D shows coil positions and corresponding output wave forms. 
  • It is observed that the waveforms obtained for coil rotations 0° - 180° is similar to that obtained in earlier case  ( Slip – ring – brush combination ). 
  • However the direction of current for coil rotation 180° - 360° is also positive which is negative in earlier case. The above fact is explained as below.  
  • The current flows through path in the direction of CDPGQAB. It means that the positive end of the supply comes in contact with ' P ' segment and negative with ' Q ' segment. 
  • The direction of the current is reversed but at the same time positions of the segments ' P ' and ' Q ' have also reversed during the next half cycle. 
  • It means that the segment ' Q ' comes in contact with positive brush and segment ' P ' with negative brush. 
  • The current flows through path BAQGPDC direction.  
  • The galvanometer reading shows that it is positive during both half cycles. i.e. Current flows unidirectional ( Direct current ). 
  • It should be noted that combination of commutator – brush function as a rectifying action i.e. alternating current into direct current. 
coil position and waveform
Effect of multi-turn coils
  • We have already understood the theoretical operation of the DC Generator. 
  • As we knows that the DC generator produces direct current, but look at the figure D, it is not pure direct current, How to obtain pure DC ? 
  • Remember that the armature is not single turn coil, it is built up by multi-turn coil.
  •  So far we are familiar with only single turn coil operation of the DC generator.
  •  Now we will try to understand the operation of DC generator for multi - turns coils. 
  • Figure E shows that the two coils are arranged at right angles to each other. When the armature rotates by 90° angles, induced voltage of one coil becomes zero and that of other coil becomes maximum and vice versa. 
two coils and corresponding waveforms

  • This will result in smoothen the DC waveforms. As we know that the armature is built up by number of coils therefore the output current is pure DC rather than pulsating.  
  • The galvanometer reading shows that it is positive during both half cycles. i.e. Current flows unidirectional ( Direct current ). 
  • It should be noted that combination of commutator – brush function as a rectifying action i.e. alternating current into direct current.
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